वृद्धि-सन्धि

This सन्धि is the substitution of वृद्धि syllables (vowels) in place of the corresponding एच् vowels - diphthongs - when they occur after अ/आ .

• अ/आ + ए/ऐ = ऐ 
• अ/आ + ओ/औ = औ 

 However, there are certain exceptions wherein गुण is only substituted for गुण vowels - ए/ओ.